Interactions between metal oxides and species of nitrogen and iodine in bioturbated marine sediments

By using a gold amalgam (Au/Hg) voltammetric microelectrode, we have measur ed simultaneously and with millimeter resolution the distributions of O-2, Mn(II), Fe(II), I(-I), and HS(-I) in bioturbated sediment cores from the La urentian Trough. We also measured nitrate and ammonia in the pore water, to tal I and ascorbate- and HCl-extractable Fe and Mn in the solid-phase sedim ent, and fluxes of O-2, NO3-, and NH4+ across the sediment-water interface. The concentrations of O-2 and Mn(II) were below their respective detection limits of 3 and 5 mu M between 4 and 12 mm depth, but a sharp iodide maxim um occurred at the depth where upward diffusing Mn(II) was being removed. W e propose that the iodide peak is maintained through the reduction of IO3- by Mn(II), reoxidation of I(-I) to IO3- in the oxic zone above the peak and oxidation to I, below where it is ultimately trapped by reaction with orga nic matter. The iodide production rate is sufficient to account for the oxi dation of all of the upward diffusing Mn(II) by IO3-. Nitrate plus nitrite (Sigma NO3) decreased to a minimum within 10 mm of the sediment-water interface, in agreement with flux measurements which showed Sigma NO3 uptake by the sediment. Below the minimum, Sigma NO3 rebounded, and reached a maximum at 40- to 50-mm depth. This rebound is attributed to the anaerobic oxidation of ammonia by manganese oxides. Fe(II) was always f irst detected below the anoxic Sigma NO3 maximum, and was accompanied by co lloidal or complexed Fe(III). A sharp upward-directed ammonia gradient was recorded near the sediment-water interface, but Ilo ammonia was released du ring the first 48 h of the incubations. If the ammonia removal were due to coupled bacterial nitrification-denitrification, more than one half of the total measured oxygen uptake (6.7 to 7.3 mmol/m(2)/d) would be required, an d more organic carbon would be oxidized by nitrate than by oxygen. This sce nario is not supported by nitrate flux calculations. Alternatively, the oxi dation of ammonia to N-2 by manganese oxides is a potential removal mechani sm. It would require one quarter of the total oxygen Bur. The high-resolution profiles of redox species support the conceptualization of bioturbated sediments as a spatially and temporally changing mosaic of redox reactions. They show evidence for a multitude of reactions whose rela tive importance will vary over time, and for reaction pathways complementin g those usually considered in diagenetic studies. Copyright (C) 2000 Elsevi er Science Ltd.